Optical device, such as a dye laser, employing a free-flowing liquid stream

ABSTRACT

There is disclosed a dye laser employing a free-falling stream of dye solution so that no laterally constricting transparent cell is needed. The solution is pumped through a nozzle shaped to provide a smooth-surfaced central portion in a ribbon-shaped flow. The shape in the output plane of the nozzle primarily determines this flow. In a typical embodiment, the solution employs a viscous solvent such as ethylene glycol to promote a smooth-surfaced flow. The solution is filtered to eliminate bubbles and particles and is recirculated.

United States Patent [191 Rosenberg et al.

[ Oct. 16, 1973 OPTICAL DEVICE, SUCH AS A DYE LASER,

EMPLOYING A FREE-FLOWING LIQUID STREAM [75] Inventors: Robert Rosenberg;Peter Klaus Runge, both of Fair Haven, NJ.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

22 Filed: Feb. 7, 1912 21 Appl. No.: 224,037

[ 56] References Cited OTHER PUBLICATIONS Arthurs et al.,Frequency-Tunable Transform-Limited Picosecond Dye-Laser Pulses, App.Phys. Let., Vol. 19, No. 11 (Dec. 1, 1971) pp. 480-482.

Primary ExaminerWilliam L. Sikes Attorney-W. L. Keefauver et al.

[57] ABSTRACT There is disclosed a dye laser employing a free-falling Istream of dye solution so that no laterally constricting transparentcell is needed. The solution is pumped through a nozzle shaped toprovide a smooth-surfaced central portion in a ribbon-shaped flow. Theshape in the output plane of the nozzle primarily determines this flow.In a typical embodiment, the solution employs a viscous solvent such asethylene glycol to promote a smooth-surfaced flow. The solution isfiltered UNITED STATES PATENTS to eliminate bubbles and particles and isrecirculated. 312311.531 511332 22K;:1:::3:31:3:1:31133333311331: 5225;?5 claims, 8 Drawing Figures 8| 84 l LASE R PU M P l a SOURCE 7 STR EAM rn 32 W RESERVO R 83 8 5 y v NOgLE l FILTER 73 75 PUMP PAIENTEDucHsms f3.7663189.

sum 1 or ,2

FIG. B I 4 Q l I PUMP] Ir I LASER I I V I5 I r STREAM :2 THIN uaum I6STREAM SOURCE FIG. 2 1

- NOZZLE Q v 3 orig me 42 ,A

sum 2 ur 42 aza f FIG. 7 PUMP T HEAT 74 2 EXCHANGER I 72 LFILTER P75/-NOZZLE f STREAM RESERVOIR 7| LASER PUMP SOURCE IT: 71 35 OPTICALDEVICE, SUCH AS A DYE LASER, EMPLOYING A FREE-FLOWING LIQUID STREAMBACKGROUND OF THE INVENTION This invention relates to dye lasersemploying flow of the active dye medium.

The dye laser is a laser that has been attracting increasing researchand development activity in the last few years because of the extremelybroad tuning bandwidths that can be achieved. In most such lasers, it isnecessary to focus the pumping light beam to a maximum extent in the dyemedium in order to reduce the oscillation threshold. The dye bleachingand other damage which results from such intense pumping is renderedunobjectionable by flowing the dye through the pumping region.

Heretofore, the dye has been flowed through the pumping region betweenalundum or glass windows through which the pumping beam passes. Thewindows SUMMARY OF THE INVENTION We have discovered that the cell thatlaterally constricts the dye flow and its transparent windows throughwhichthe pumping beam passes can be com pletely eliminated by employinga free-flowing dye stream. This stream is obtained by pumping the dyesolution through a nozzle shaped to provide a smoothsurfaced centralportion in a ribbon-shaped flow.

According to one subsidiary feature of the invention, the shape of thedischarge opening of the nozzle is made generally rectangular andelongated in one transverse dimension. The opening is rounded at thelimits of elongation to promote the stream edge conditions that favor asmooth-surfaced central portion of the flow. Simultaneous curvature ofthe limits of the opening backward in the axial direction promotes arelatively broad ribbon-shaped flow of the dye.

According to one specific feature of our invention, the dye is directedthrough the nozzle in a downward direction so that the dye solutionfalls freely in its ribbon-shaped flow through the pumping beam. Thesmooth parallel surfaces of the ribbon-shaped flow are preferablydisposed at the Brewster-angle with respect to the pumping beam.

According to another specific feature of our invention, the dye solutionemploys a viscous solvent such as ethylene glycol. Upon falling into areservoir, the solution is pumped through a filter to eliminateparticles and bubbles and is recirculated to the nozzle.

Our invention is broadly applicable to the use of a liquid in anyoptical system in which low loss is desirable.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially block diagrammaticillustration of a first embodiment of our invention;

FIG. 2 is a pictorial view of a preferred nozzle and the resultingribbon-shaped flow according to our invention;

FIG. 3 shows an enlarged cross-sectional view of the dye stream of FIG.2;

FIGS. 4 and 5 show pictorial views, partially in section, of twodifferent elevations of the nozzle of FIG. 2;

6 FIG. 6 shows an elevation view of a modified nozzle, partially insection;

FIG. 7 is a partially pictorial and partially block diagrammaticillustration of the complete dye flow apparatus for the embodiments ofFIGS. 1 and 8; and

FIG. 8 shows a plan view of a modification of the embodiment of FIG. 1,including the complete dye flow apparatus of FIG. 7.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the illustrativeembodiment of FIG. 1 is shown a discovery that eliminates the dye cellfrom an intracavity pumped continuous-wave flowing dye laser.Additionally, it is found that this dye laser may also be readily modelocked.

In the laser of FIG. 1 the pumping laser source 11 and the free-fallingdye stream 12, shown in cross section in the plan view, share the sameoptical resonator. The resonator comprises the reflectors l3, l4 and 15.Reflectors 13 and 14 have curvatures suitable for maintaining asubstantially collimated pumping light beam in the pump laser 11.Simultaneously, reflector 14 cooperates with reflector 15, which has arelatively small radius of curvature as compared to reflector 13, tofocus the pumping light beam to as small a waist as possible within thestream 12.

The stream 12 is directed downward through the pumping light beam at itswaist from a source 16, which is disposed above the waist of te pumpinglight beam. The flow issues from a nozzle which is described more fullyhereinafter in connection with FIGS. 2 and 4 through 6. The stream 12has a ribbon-like central portion of its cross section, as promoted bythe nozzledischarge shape, even thoughits edge portions assume asomewhat tubular appearance because of surface'tension effects. Thesubstantially planar and parallel smooth'surfaces of the ribbon streamare disposed with respect to the axis of the pumping light beam atsubstantially the well-known Brewsters angle. That angle lies in theplane of folding of the resonator, as defined by the two segments of theoptical path of the resonated'light. It may be noted that the beam waistof the pumping light is also sometimes referred to as the focal regionof the pumping light.

Stability, in the sense'of freedom from surface waves, in thesubstantially parallel surfaces of stream 12 traversed by the resonatedlight, has been achieved by employing the viscous solvent ethyleneglycol.

The most efficient use of the ribbon stream of thedye achieved so farhas resulted from the use of a nozzle 21 of the type shown in FIG. 2. InFIG. 2 it is seen that the discharge opening 22 of the nozzle has firstbeen pinched down to have parallel flanges 41 and 42 which are in turnparallel to the intended surfaces of the ribbon stream and has furtherbeen opened slightly up the sides, that is, backward along the axis offlow, by a shallow slit which has ends of smooth curvature.

While more complicated nozzles have been built with internal transitionsections, it has been determined that such transition sections precedingthe discharge opening of the nozzle do not improve the flowcharacteristics and may actually impair them by allowing turbulence andresulting bubbles to build up before discharge.

In the operation of the embodiment of FIG. 1, it has been found that theelimination of the dye cell walls removes several operating problems.Losses and distortion introduced by the windows of the prior art cellsare eliminated; and the apparatus becomes more reliable and apparentlylonger-lived. In fact, in our preliminary experiments, the windowlessarrangement has resulted in a threefold increase in avilable pump powerinside the cavity and in much improved long-term stability in amode-locked cresyl violet dye laser. The cresyl violet dye solution modelocked the helium-neon pumping laser, as well as itself providing thestimulated emission of coherent radiation. With the mode-lockedheliumneon pumping laser, the resulting dye laser pulses are inherentlymode locked.

The initial operation has tended to indicate increased useful lifebecause there is no window contamination by damage dye molecules andother agents; and there is no dissolving of the cement which holds thewindows in the cell. Certainly, nearly all problems associated with thewindows are eliminated by eliminating the windows.

The following modifications of the invention are feasible. In one of ourearly experiments, a nozzle was made by casting epoxy into a mold madefrom a brass rod which was tapered to the desired shapes of the internaltransition and the discharge opening of the nozzle, so that the rodlooked like a screwdriver blade, and inserted into a brass tube whichallowed sufficient clearance for epoxy to be injected between the rodand the tube. Before the epoxy was injected, the rod was sprayed with arelease agent. After curing of the epoxy,

the rod was removed and the aperture of the nozzle was carefullyfinished to provide a smooth exit shape for the stream of dye solution.

Another alternative nozzle was then made, as shown in FIG. 6, bycementing two razor blades 61 and 62 to the end of a brass tube 63 likethat previously used for the outer shell of the mold. The cutting edgesof blades 61 and 62 were separated by a distance of the order of 1millimeter. Surprisingly, it was found that the flow from this nozzlewas superior to that from the nozzle having the tapered interiortransition section.

Further illustrative detail of a nozzle similar to that of FIG. 2 isshown in FIGS. 4 and 5. In FIG. 4 one sees a side view of the nozzle,partly in cross section, clearly showing the parallel flanges 41 and 42and showing in full the shape of a rounded end 43 of the opening 22extending backward along the side of the nozzlein the axial flowdirection.

It should be noted that the axial length of the transition section inthis nozzle should be optimal so as to broaden the flat portion of theflow without introducing surface waves thereon. The transition sectionmay be regarded as that portion of the, tube 40 extending axiallybetween the portion of the normal cylindrical shape to the flanges 41and 42.

FIG. 5 shows another elevation of the same nozzle, partly in section,viewed at right angles to the elevation of FIG. 4. In this view it isclearly seen that the flange 42 and also the flange 41 which is notvisible beneath it, flatten and widen substantially as a result of beingpinched together. The curvature of the edges of flange 42 in thiselevation is not critical and can be fairly well confined to the edgeportions thereof in order to make the axial length of the flanges 41 and42 as short as possible.

Even though the ribbon stream of the dye solution falls freely throughthe air, it is not necessary to discard it after it has passed throughthe beam of pumping light. As shown in FIG. 7, it can be captured in areservoir 71 and pumped by a pump 72 through a pick-up line 73 andfurther through a heat exchanger 76 and another connecting line 74 to afilter 75. The filter 75 is designed primarily to eliminate particlesand the bubbles tht tend to form in the viscous solvent of the stream32. The filtered solution is thereafter recirculated to nozzle 21 andejected from its output portion 22 to continue the ribbon stream.

The implementation of this continuous recirculation of the dye solutionin the embodiment of FIG. 1 is illustrated in the modified embodiment ofFIG. 8. FIG. 8 further illustrates the principle that the dye streamneed not be placed within the pumping laser cavity but may have its ownresonator separate from the laser pump source. That laser pump source isdesignated 81 in FIG. 5 and may be of any conventional type suitable forpumping the dye solution. The dye laser resonator comprises reflectors83, 84 and 85, which may be substantially identical to the reflectors13, 14 and 15 of FIG. 1, except that they need have high reflectivityonly over the dye laser band. Indeed, it is advantageous that reflector83 have relatively high transmission and low reflectivity for thepumping light and that at least reflector 84 alone be a truly broadbandreflector encompassing both the pumping laser linewidth and thepotential tuning band of the dye laser. Other components of theembodiment of FIG. 8 are numbered the same as in the preceding figures.

For the specific case in which the dye is cresyl violet dissolved inethylene glycol, it was found advantageous to pump the dye with a I-IeNepump laser 11 or 81. Other viscous solvents should also be attractivefor use in the present invention. The ethylene glycol employed by us hada viscosity of 20 compared to a viscosity of unity for water. Surfacewaves of the ribbonstream 32 may also be avoidable with a solventviscosity significantly lower than 20. In other words, a viscosity aslarge as 20 is not considered critical to the present invention.

While no specific means are shown for tuning or mode-locking theselasers, it should be clear that any of the conventional means may beused, since they do not interfere with the ribbon stream of the dyesolution. In fact, because of the reduced losses, mode-locking may beachieved more readily and the tuning bandwidth should be somewhatimproved. We have been able to triple the available pump power byemploying our invention to replace prior art dye cells.

With respect to mode-locking of any lasers, it should be appreciatedthat saturable-absorbing solutions of dyes, which act passivly on thestimulated emission in the resonator of a laser, are among the bestmodelocking devices. Use of our invention in place of a mode-lockingcell will promote increased pulse energy and an associated reducedpulsewidth of the modelocked pulses.

It should be clear that our invention can be used to reduce losses ofany liquid solution, such as a dye solua substantial reduction incross-sectional area and k a cross-sectional shape that is elongatedtransverse to and substantially at Brewsters angle to said optical path,and

means for pumping the liquid at a variable speed of flow in said freeflow region,

said nozzle and said pumping means together providing a stable ribbonshape of said flow without the aid of any material constraints of saidflow where it intercepts the optical path, and

means for supplying coherent optical radiation in said optical paththrough said liquid.

2. A coherent optical device according to claim 1 in which the nozzlehas a transition from the constant cross-sectional area passageway tothe free flow region that is characterized by essentially a stepwisereduction in cross-sectional area and negligible length along the liquidflow path.

3. A cpherent optical device according to claim 1 in which thecirculating means includes, between the pumping means and the nozzle,means for filtering particles and bubbles from the liquid before itpasses' through said nozzle.

4. A coherent optical device according to claim 1 in which the viscosityof the liquid is of the order of 20.

5. A coherent optical device comprising a liquid selected fordisposition in the optical path of said device, means for circulatingsaid liquid through a flow path including a section in which the liquidflows free of contact forces, including a nozzle having a passageway ofsubstantially constant cross-sectional area, a substantiallyturbulence-free and bubble-free transiton from said constantcross-sectional area passagway to said free-flow section, and having anexit of elongated cross-section at said free-flow section to produce anessentially ribbon-shaped flow of said liquid in said section, and meansfor supplying coherent optical radiation through a central portion ofsaid ribbon-shaped flow, the liquid being a solution of a dye capable ofabsorbing a portion of the supplied radiation, the supplying meanssupplying linearly polarized radiation, and the nozzle being oriented todispose the smooth surfaces of the ribbon-shaped flow at Brewsters anglewith respect to the direction of propagation of the polarized radiation.

2. A coherent optical device according to claim 1 in which the nozzlehas a transition from the constant cross-sectional area passageway tothe free flow region that is characterized by essentially a stepwisereduction in cross-sectionaL area and negligible length along the liquidflow path.
 3. A coherent optical device according to claim 1 in whichthe circulating means includes, between the pumping means and thenozzle, means for filtering particles and bubbles from the liquid beforeit passes through said nozzle.
 4. A coherent optical device according toclaim 1 in which the viscosity of the liquid is of the order of
 20. 5. Acoherent optical device comprising a liquid selected for disposition inthe optical path of said device, means for circulating said liquidthrough a flow path including a section in which the liquid flows freeof contact forces, including a nozzle having a passageway ofsubstantially constant cross-sectional area, a substantiallyturbulence-free and bubble-free transiton from said constantcross-sectional area passagway to said free-flow section, and having anexit of elongated cross-section at said free-flow section to produce anessentially ribbon-shaped flow of said liquid in said section, and meansfor supplying coherent optical radiation through a central portion ofsaid ribbon-shaped flow, the liquid being a solution of a dye capable ofabsorbing a portion of the supplied radiation, the supplying meanssupplying linearly polarized radiation, and the nozzle being oriented todispose the smooth surfaces of the ribbon-shaped flow at Brewster''sangle with respect to the direction of propagation of the polarizedradiation.